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Research studies the nutritional and technological effect of polishing and parboiling pigmented rice

7 October 2015

As a cereal grain, rice is the most widely consumed staple food for a large part of the world's human population, and mainly consumed as polished rice. This polishing process is performed on the rice so consumers can benefit from improved physical, sensory and shelf life properties. The polishing technique removes the bran and the germ of the rice caryopsis, which is detrimental to the nutritional qualities of rice as the bran layers are rich in proteins, vitamins and fat, whilst the germ is mainly fat.

A common secondary processing technique is parboiling, effective in increasing the storage stability of rice. The three stages of parboiling are: the transfer of bran components to the inner layers of the rice grain during soaking, the inactivation of lipases due to heat treatment, and starch gelatinisation at the drying stage.

A study performed by the Department of Agroindustrial Science and Technology, at the Federal University of Pelotas in Brazil, used specific strains of black and red rice grains. The polishing treatment caused a 16.2% decrease in protein of black rice and 6.2% in red rice. Further to this, the soaking stage of parboiling decreased the protein content in both types of rice further. Polishing the rice also caused a drop in fat content of 86.2% and 77.4% in black and red rice respectively, the cause of this being from removing the germ of the grain which has a high fat content. Parboiling the grains also caused a further decrease in fat content of over 10%. The researchers hypothesised that this occurred as detached oil bodies within the grain leaching out during the hydration of the rice. Additionally, the polishing treatment removed 92.6% and 97.5% of antioxidant phenolic content of black and red rice respectively.

It is food for thought that the rice processing industry has to remove desirable nutritional benefits in order to meet the physical and sensory properties acceptable to consumers.

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